This invention generally concerns optical amplifiers, and is particularly directed to a method of distributing loss among two or more optical amplifiers to reduce overall noise accumulation from an amplifier site.
Optical amplifiers for amplifying photonic signals transmitted through optical fiber networks are well known in the art. Such amplifiers are used to extend transmission distances and to compensate for losses from various network elements. Presently, there are several known types of optical amplifiers, including erbium-doped fiber amplifiers (EDFAs), and Raman amplifiers.
EDFAs typically comprise at least one pump laser whose output is optically coupled to at least one coil of erbium-doped optical fiber. In operation, the output of the pump laser excites the atoms of the erbium-dopant within the fiber. These excited atoms release their excess energy in proportion to the strength of the incoming optical signal, which results in an amplified output. By contrast, Raman amplifiers achieve amplification without the need for erbium-doped optical fibers. Therefore, Raman amplifiers may use optical fibers without Er dopant as their gain fiber.
In one type of Raman amplifier, the output of a pair of orthogonally polarized pump-diode lasers provides backward propagating pump power in the gain fiber. Alternatively, a single pump and a de-polarizer may also be utilized to provide pump power in the gain fiber. Forward-propagating signals are amplified in the gain fiber by higher energy (shorter wavelength) pump photons scattering off the vibrational modes of the optical fiber""s lattice matrix and coherently adding to the lower-energy (longer wavelength) signal photons.
Raman amplifiers may be one of two types, depending upon the type of the gain fiber used therein. Distributed Raman amplifiers advantageously use the optical transmission fiber itself as the gain fiber. By contrast, discrete Raman amplifiers utilize their own optical fiber as the gain fiber. While the dopant used in the gain fiber of a discrete Raman amplifier is typically the same as used in the optical transmission fiber (e.g., germanium), the gain fiber of the discrete Raman amplifier usually contains higher concentrations of dopant (such as germanium) than a conventional optical transmission fiber and is designed to operate with a decreased fiber effective area in order to provide gain fiber with a a higher non-linear coefficient.
Furthermore, with the increasing demand for more data traffic, more transmission system providers are interested in providing high bit rate (40 Gbs, or higher) transmission systems. For transmission systems operating at a high bit rate, Signal to Noise ratio (S/N) becomes more of a problem than for the slower transmission systems. Therefore, managing generation and accumulation of optical noise in in transmission systems is very important. This optical noise is generated by the amplification process itself (for example, amplified spontaneous emission (ASE) noise).
One way of improving the Signal to Noise ratio in a high bit rate transmission system is to utilize the distributed Raman amplifiers in conjunction with EDFAs or other discrete optical amplifiers. Distributed Raman amplifiers distribute amplification along the length of the transmission fiber, resulting in a lower optical noise contribution from the optical amplifier sites (also referred to as huts). More specifically, the optical noise becomes attenuated because the amplification is spread over the length of the transmission fiber and is not concentrated at the site location. An optical amplifier site or hut includes one or several discrete optical amplifiers situated proximately to one another and coupled to the transmission fiber. It may also include a Raman pump module for the distributed Raman amplifier which comprises one or more pump sources; pump power or temperature control unit(s); environmental change sensor(s) or other optical components such as couplers, connectors or taps.
An optical An optical amplifier site connected to a distributed Raman amplifier fiber, the optical amplifier site comprising:
(i) a distributed Raman amplifier pump module, connected to the distributed Raman amplifier fiber, such that the Raman amplifier fiber and the pump module form a distributed Raman amplifier that provides a first signal gain level, the first gain level being greater than 10 dB;
(ii) an optical amplifier located downstream from the distributed Raman amplifier, the optical amplifier being indirectly coupled to the distributed Raman amplifier, the optical amplifier including a first amplification medium, providing a second signal gain level;
(iii) at least one active optical signal loss element located between the distributed Raman amplifier and the optical amplifier, the at least one loss element contributing to optical loss of the amplifier site and providing at least 0.5 dB loss;
(iv) a second amplification medium located down stream from the first amplification medium;
(v) at least another one active optical signal loss element, the at least another one optical signal loss element being located between the first amplification medium and the second amplification medium.
One advantage of the present invention is the reduction of noise of the transmission systems and to increase in performance of amplifier sites.